1. Field of the Invention
The present invention relates to an oxygen sensor and a method for driving the same and, more particularly, to a wide range air to fuel (A/F) ratio sensor utilizing the characteristics of oxygen concentration and oxygen pumping cells made of a solid electrolyte such as YSZ (Y.sub.2 O.sub.3 stabilized ZrO.sub.2) and a method for driving such a wide range A/F ratio sensor.
2. Description of the Prior Art
Oxygen sensors utilizing the characteristic of oxygen concentration cells made of a solid electrolyte such as YSZ are being widely used for the purpose of, for example, controlling the A/F ratio in automobiles. Recently, developments have been made in wide range A/F ratio sensors which utilize the oxygen ion migration characteristic of a solid electrolyte and the oxygen concentration cell characteristic. Such wide range A/F ratio sensors have an advantage that they can be used to control the air to fuel ratio over a wide range from fuel rich to fuel lean.
However, conventional wide range A/F ratio sensors have a significant problem in that their manufacture is difficult due to their complex structure. Furthermore, it is difficult to ensure the reproducibility. FIG. 1 illustrates schematically a typical wide range A/F ratio sensor being used in an actual situation. As shown in FIG. 1, the A/F ratio sensor includes an oxygen pumping cell 1 and a sensing cell 3 both laminated independently of each other. Between the cells 1 and 3, a porous diffusion passage is provided which defines a diffusion chamber 2. Due to such complex structure, the A/F ratio sensor is difficult to manufacture (Society of Automobile Engineers 920234).
In particular, this A/F ratio sensor requires a time interval for sending a variation in the concentration of gas, occurring upon pumping oxygen, to the sensing electrode. Such a time interval results in a delayed response of sensing and an increase in the operation pumping voltage over a required level. In addition, the A/F ratio sensor has a structural limitation. In other words, the space between the sensing electrode and the pumping electrode must be appropriately adjusted.
On the other hand, FIG. 2 illustrates another A/F ratio sensor suggested in U.S. Pat. No. 4,776,943. This patent describes that the problem involved in the A/F ratio sensor structure of FIG. 2 may be eliminated by adapting the appropriate AC circuit. In the 4,776,943 patent, however, no concrete mechanism or method is taught. Moreover, since this A/F ratio sensor has a pumping electrode 12 exposed to air as shown in FIG. 2, a voltage is generated due to an oxygen partial difference between pumping electrodes 11 and 12 even when no pumping voltage is applied. This A/F ratio sensor also has a fundamental problem of requiring the inevitable application of surplus pumping voltage upon measuring a lean region. Due to such problems, it is difficult to use this A/F ratio sensor as a wide range sensor.